Such devices are known, for example servo phase loops generally referred to as phase locked loops (PLL).
A device of this type comprises:                an input intended to receive an electrical signal oscillating at a reference frequency,        an output intended to provide an electrical signal oscillating at an output frequency,        a servo circuit for the control of the output frequency by the reference frequency, connecting the input to the output of the device and comprising a phase comparator, a loop filter and a controlled frequency oscillator providing the electrical signal oscillating at the output frequency, and        a feedback loop connecting the output to one of the two comparison inputs of the phase comparator.        
The controlled frequency oscillator, for example of the VCO (Voltage Controlled Oscillator), DCO (Digitally Controlled Oscillator) or other type, is controlled by a digital value, a voltage or an analog current, or even a combination of an analog value and digital value. Although this is generally not the case, the transfer function of such an oscillator is often considered as linear and simply represented by a conversion factor K.
The phase comparator operates in general on the edges of two signals to be compared, i.e. when these signals reach the same value at the same slope. The phase comparator then itself provides one or several signals that represent the phase difference between the edges of the two compared signals. Most of the time, the signals resulting from the comparison are transformed into a single current or voltage pulse signal by a charge pump. This current (or voltage) is of a constant amplitude I (or U), it takes the sign of the phase difference and its pulse has a width that is proportional to the phase difference. The phase comparator can be carried out in a more or less analog or digital manner.
The pulse provided by the charge pump is then filtered by the loop filter which has as a base the pulse response of an integrator filter. The loop filter can also be carried out in a more or less analog or digital manner.
The result coming from the loop filter is then applied as a control of the controlled frequency oscillator. As the loop filter is an integrator and the integral of the phase of a periodic signal gives its frequency, the control of the oscillator is indeed proportional to a frequency that will move towards the desired frequency as output as the phase difference with the reference signal will move towards 0 or towards another constant value.
The PLL are as such generally used in electronic circuits as sources of high frequencies. Indeed, these devices make it possible using a source with a low frequency and a high spectral purity (for example quartz emitting periodic signals at a few MHz) to obtain high-frequency periodic signals (for example a few GHz) and with a spectral purity of better quality than devices that directly generate high-frequency signals.
Concretely, for a source of reference frequency that is low and of high spectral purity Fref, a high-frequency signal with good spectral purity Fc=α·Fref is obtained as output, a being a multiplication factor chosen greater than 1. This multiplication factor α is generally variable and of a non-integer real value so as to vary the various channels of standards used according to the application.
A major parameter of frequency synthesis devices is the time Δt for establishing their operating regime, i.e. the time that they take to be operational, either at start-up, or during a change in the channel (i.e. a change in the factor α). This time Δt lasts during a transient regime, generally qualified as pull-in time, preceding the operating regime. As such for example, during the passing ΔFc from a frequency Fc to a frequency F′c, the transient regime of synthesized frequency follows an exponential envelope that moves asymptotically towards F′c at a natural resonant frequency w during the duration Δt. The duration Δt of the pull-in time depends on parameters that constitute a frequency synthesis device and limit its reactivity.
Another major parameter of frequency synthesis devices is the resolution of the possible variations of the multiplication factor α and therefore the fineness of the adjustment that is possible on the output frequency Fc according to the target applications or standards.
Other parameters to be considered, in order to optimize frequency synthesis devices and the quality of the periodic signals of frequency Fc obtained as output, include phase noise, jitter, spurious signals and electrical consumption.